How much bigger than bore diameter can a fixed loaded cast bullet be without causing leading problems?  What is the ultimate relationship to the bore diameter compared to the nose riding section of a bullet?  I am using 20:1 alloy and am just amazed at how accurate my 40-70 SS is with a breech seated bullet and 21 grains of IMR 4227 and am now thinking of getting an Accuratemolds built mould for a fixed load.  Thanks.  Tom

Generally, oversize bullets do not cause leading. Undersize bullets do,
Fred.

I personally have never competed with fixed but from what little I do understand it will never be as good as being breech seated. You just might have the bullet that you need and your expectations maybe just a little bit high but don't hold me to it. I would also encourage you to some day take your 25-35 out and to breech seat it. Just seat until it engraves just at or beyond the base and don't worry about how it is chambered and I think you will be quite surprised with your results. If you choose to do so someday I can provide you with some load information to get you started with.

JLouis wrote on Jul 16^th, 2020 at 4:21pm:


The only head-to-head matches between  fixed ammunition and shhuetzen rifles is in the cast bullet Association. If you look at the scores in the records they are very similar.
The heavy and unrestricted  benchrest guns we use bump/swage  dies so the bullet  completely fits the chamber similar to what breech seating does.
Similar to your advice  is just all about bullet fit.

Tom it is unfortunate that you cannot breech seat the 25-35 but is outstanding that it shoots fixed so well. The only load data I have is for breech seating I have never shot any fixed so I can't be of any help.

Some of us have long used cast 205gr GC bullets sized 0,329" in 8X58 caliber Swedish rollers with 0,323" to 0,326" or larger groove to groove bores with no leading and good to excellent groups for Metallic Sil., etc. (muzzle velocities about 1500 fps) In doing so we were following an ancient habit of using such "oversize" lead bullets to better fill the "leed", thereby providing better alignment of bullet with bore. 

Equally good, sometimes better, groups could be obtained with a NOE #326378 250 gr. RN GC, with the grease groove part of bullet sized a bit larger than groove to groove diameter or at leed diameter. This bullet has the classic, ancient long, slightly tapered nose that extends some distance into the rifling, usually giving excellent groups.  Some popular BPCR bullets operate on same principal and called "Money Bullets" .

Good "bullet to leed" fit has long been advantagious.

SKG

I prefer bullet diameter to match groove diameter exactly or smaller. 

If I don't get the fit I want I buy barrels till I get one that fits the bullet.

westerner wrote on Jul 19^th, 2020 at 5:43pm:


Written by a realist that knows that it is the bullets that make the holes in the target, that get scored. Everything else is just "process".

SKG